Nuclear Magnetic Resonance Characterization and Analytical Modeling of Compressibility of Propped Fracture in Coal: New Insights

A proper understanding of the compressibility characteristics of propped fractures is crucial to the accurate prediction of coalbed methane production and the proper deployment of development strategies due to the application of hydraulic fracturing. In this article, online nuclear magnetic resonance experiments and improved analytical models were integrated to characterize the variable compressibility of propped fractures in coal. Experimental results demonstrated that a higher concentration or smaller size of proppant results in a lower compressibility of the propped fracture. It is also shown that the compressibility of propped fracture exhibits an obvious stress-sensitive behavior. Specifically, with increasing effective stress, the compressibility of proppant fracture gradually decreases, and the decrease is significant at low effective stress but gradually stabilizes at high effective stress. Analysis based on the proposed model showed that increasing the proppant concentration is an effective method to reduce the impact of the Young’s modulus of coal on the compressibility of propped fracture, while the impact of Poisson’s ratio of coal on the compressibility of propped fracture is negligible. Moreover, a larger Poisson’s ratio and Young’s modulus of proppant can effectively reduce the compressibility of propped fracture. The stress sensitivity of propped-fracture compressibility increases significantly with an increase in proppant concentration or a decrease in proppant size, whereas the effects of mechanical properties of coal and proppant are negligible.